Constant voltage and constant frequency (CVCF) inverters having a LC filter at the a.c. output are used as uninterruptible power supplies or stand alone power supplies for delivering power to equipment in which instantaneous service interruption is not permitted, such as a computer, etc. In most cases, there is employed, as the load, a non-linear load from which an output voltage waveform distortion is generated in synchronization with the output voltage period, such as, a d.c. load to which a power is supplied through the rectifier.
To eliminate the output voltage waveform distortion of the inverter produced by such a non-linear load, there have been proposed a method of detecting an output voltage to control the inverter output on the basis of an instantaneous value of a deviation between the output voltage detected and an output voltage reference so that the deviation becomes small, a method of allowing a current flowing in the capacitor of the LC filter to follow a sine wave, and the like.
Referring to FIG. 1, there is shown an apparatus in which the method of controlling the inverter so that a current flowing in the capacitor of the LC filter follows a sine wave is employed among the above methods. In this FIGURE, a d.c. output of a d.c. power supply 1 is smoothed by a smoothing capacitor 2, and is then transformed to a.c. by a three-phase inverter 3. This a.c. is transformed to a suitable voltage by an output transformer 4, and is then delivered to a load 7 through an LC filter consisting of a reactor (L) 5 and a capacitor (C) 6. The LC filter is provided in order to eliminate higher harmonics produced by the switching of the inverter 3 to deliver an a.c. voltage having less distortion in the load 7.
The current flowing in the capacitor 6 is detected for each phase by a current detector 8, and current detection signals I.sub.cu, I.sub.cv, I.sub.cw obtained as the detected result are inputted as the feedback signals to the inverter 3. Furthermore, a current reference generator 9 prepares three phase reference currents I.sub.cu *, I.sub.cv *, I.sub.cw * on the basis of the reference voltage V* and the reference phase .theta.*. EQU I.sub.cu *=.omega.C.multidot.V* cos(.theta.*) EQU I.sub.cv *=.omega.C.multidot.V* cos(.theta.*-2n/3) EQU I.sub.cw *=.omega.C.multidot.V* cos(.theta.*+2n/3) (1)
where .omega. is an output angular frequency (rad/sec) of the inverter 3, and C is a capacitance (F) of the capacitor 6. When the time is assumed as t, .theta.* is expressed as follows: EQU .theta.*=.omega..multidot.t (2)
Deviations between these current references and the current detection signals are respectively obtained by adders 10U, 10V, 10W as follows: EQU .DELTA.I.sub.cu =I.sub.cu *-I.sub.cu EQU .DELTA.I.sub.cv =I.sub.cv *-I.sub.cv EQU .DELTA.I.sub.cw =I.sub.cw *-I.sub.cw
These deviations are delivered to a gate control circuit 11. A gate control circuit 11 prepares a pulse width modulation (PWM) pattern so as not to exceed above the hysteresis level .+-..DELTA.I* as shown in FIG. 2 (only U-phase is illustrated) in order to reduce these current deviations .DELTA.I.sub.cu, .DELTA.I.sub.cv, .DELTA.I.sub.cw to control the inverter 3 using the PWM pattern as a gate signal.
In the conventional control apparatus, for eliminating such an output voltage, there are employed a method to detect a current flowing in the filter capacitor to effect closed loop control so that the difference between the current detected and the reference current becomes small, and a method to detect an inverter output voltage to effect closed loop control so that the deviation between the voltage detected and a reference voltage becomes small, and the like. However, with these conventional control apparatus, it is difficult to sufficiently remove the waveform distortion as a result of the time delay until a control command is outputted from the detection. If one persists with such conventional control methods, a control system having a very fast response characteristic until a command is given from the detection of the output voltage, or the output current must be used.